Don't call simplify_aggref with a NULL PlannerInfo

42473b3 added prosupport infrastructure to allow simplification of
Aggrefs during constant-folding.  In some cases the context->root that's
given to eval_const_expressions_mutator() can be NULL.  42473b3 failed
to take that into account, which could result in a crash.

To fix, add a check and only call simplify_aggref() when the PlannerInfo
is set.

Author: David Rowley <dgrowleyml@gmail.com>
Reported-by: Birler, Altan <altan.birler@tum.de>
Discussion: https://postgr.es/m/132d4da23b844d5ab9e352d34096eab5@tum.de

Author: david-rowley

src/backend/optimizer/util/clauses.c

@@ -2638,7 +2638,9 @@ eval_const_expressions_mutator(Node *node,
 			}
 		case T_Aggref:
 			node = ece_generic_processing(node);
-			return simplify_aggref((Aggref *) node, context);
+			if (context->root != NULL)
+				return simplify_aggref((Aggref *) node, context);
+			return node;
 		case T_OpExpr:
 			{
 				OpExpr	   *expr = (OpExpr *) node;